2023 A/L Physics Past Paper · Question 03 (English Medium)

Answer: (4) The force exerted on the bull by the ground

What the question asks

We want: the force that makes the bull move forward.

What each letter means:

$B$: the bull.

$C$: the cart.

$G$: the ground.

$F_{XY}$ ($\text{N}$): the force on $X$ exerted by $Y$. For example, $F_{BG}$ is the force on the bull exerted by the ground.

Relevant theory

What is happening:

The bull walks forward and pulls the cart along a road.

The bull touches the cart through the harness, and its feet touch the ground.

The bull pulls the cart forward. The cart pulls the bull backward. The feet push the ground backward. The ground pushes the feet forward.

We want the one force that acts on the bull and points forward.

When the objects interact (collide), they exert equal and oppsite forces on one another.

The wall pushes back on you with an equal and opposite force.

Friction is the force that tries to stop materials sliding across each other. Frictional force prevents or opposes relative motion.

Reading the options

For example, think of walking on a path and then on slippery ice.

On the path you push the ground backwards with your foot. The ground pushes you forwards, and you walk on.

On ice your foot slides backwards and it is hard to walk. The ice cannot push you forwards well.

The bull is the same. It cannot push itself forward. It pushes the ground backward, and the ground pushes it forward.

The first quote says that two bodies that interact exert equal and opposite forces on one another. The second quote gives the same idea with a wall.

The third quote says that friction opposes sliding. The feet would slide backwards over the ground if there were no friction, and friction stops that. So the forward push of the ground on the bull is friction.

$$F_{GB} = F_{BG} \quad\text{in size, and opposite in direction}$$

In words: the push of the bull on the ground and the push of the ground on the bull are a pair. They are equal in size and opposite in direction.

Ask two things about each option: on which body does the force act, and which way does it point?

(1) $F_{CB}$ acts on the cart, forwards. It cannot move the bull.

(2) $F_{BC}$ acts on the bull, but backwards. It holds the bull back.

(3) $F_{GB}$ acts on the ground, backwards.

(4) $F_{BG}$ acts on the bull, forwards. It is the partner of (3). This is the one.

(5) $F_{GC}$ acts on the ground.

Only option (4) acts on the bull and points forward.

Why the other options fail

This force acts on the cart, not on the bull.

This force acts on the bull, but it is equal and opposite to the bull's pull on the cart. It points backwards.

This force acts on the ground. It is the partner of the ground's push on the bull.

This force acts on the ground, so it cannot move the bull.

Conclusion

The ground pushes the bull forward. This force moves the bull.

The answer is (4).

To find what moves a body, look for a force that acts on that body.

A force that the body exerts on something else acts on that other thing.

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